1. Technical Field
The present invention relates to a satellite communication system operated under Demand Assignment Multiple Access system (hereinafter referred to as DAMA system) for the purpose of communications between a hub station and a plurality of remote stations, and in which common signal channel line (hereinafter referred to as CSC line) from remote stations to a hub station is used for data transmission.
2. Background Art
FIG. 5 is a schematic diagram showing an arrangement of one of conventional satellite communication systems operated under the DAMA system. DAMA system stands for a system in which a line required for communication is assigned on demand, in a communication network consisting of a plurality of earth stations. For that purpose, the DAMA system is provided with two types of lines, i.e., CSC line and communication line respectively. The CSC line is used for demanding assignment of a communication line and for controlling assignment of an idle line, and the assigned communication line is used for actual communication.
Referring to FIG. 5, reference numeral 31 is a hub station engaged in total control of a network including assignment, supervision and control of lines by the DAMA system, in the mentioned conventional satellite communication system. Numeral 32 is a plurality of remote stations, and numeral 33 is a communication satellite launched into space for relaying communication lines. Numeral 34 are forward CSC lines from the remote stations 32 to the hub station 31 through the communication satellite 33. In the operation of the DAMA system, the CSC lines 34 are used for transmission of demands for assignment of lines, supervisory signals and reply signals to the supervisory signals, or the like. Numeral 35 are backward CSC line from the hub station 31 to the remote stations 32 through the communication satellite 33. Numeral 36 is communication lines used for actual communications.
FIG. 6 is a diagram showing an arrangement of frequency band and time slots of the forward CSC line 34 and backward CSC line 35, fixed by the mentioned satellite communication system. Referring to FIG. 6, numeral 35 is a frequency band of the backward CSC line from the hub station 31 to the remote stations 32, and numeral 34 is a frequency band of the forward CSC line from the remote stations 32 to the hub station 31. Further, a different frequency band is allocated to the satellite communication lines 36, though not shown in FIG. 6.
FIG. 7 is a block diagram showing an arrangement of the conventional hub station 31. In the drawing, numeral 40 is a terminal unit. Numeral 41 is a line connection control device. This line connection control device 41 consists of: a DAMA control section 41a for detecting and assigning idle lines in response to demands for line assignment from the remote stations 32; and a supervision and control section 41b for transmitting supervisory signals to check whether or not the operation of remote stations is normal and receiving reply signals from the remote stations. Numeral 42 is a CSC modulator section for modulating radio signals such as instructions of line assignment from the line connection control device 41 to a frequency band of the backward CSC line 35. Numeral 43 is a CSC demodulator section for demodulating received signals corresponding to the frequency of the forward CSC line 34 shown in FIG. 6. Numeral 44a is a communication line modulator/demodulator (modem) section of the hub station 31 for establishing a frequency on an idle communication line for actual communication in accordance with instructions from the DAMA control section 41a, and for transmitting/receiving radio signals of the mentioned frequency. Numeral 45 is a transmitter/receiver for transmitting radio signals outputted from the CSC modulator 42 into space after amplifying the signals at the time of transmission and, and for converting radio signals received from the satellite 33 into intermediate frequency signals and outputting those signals to the CSC demodulator 43 at the time of reception.
FIG. 8 is a block diagram showing an arrangement of the conventional remote station 32. In the drawing, numeral 50 is a terminal unit. Numeral 51 is an IDU (In Door Unit). This IDU 51 consists of a CSC modulator/demodulator (modem) section 51a corresponding to the frequency of the forward CSC line 34, a communication line modulator/demodulator (modem) section 44b of the remote station 32 for establishing a frequency on an idle line at the time of communication and transmitting/receiving radio signals of the mentioned frequency, and an IDU control section 51c for controlling the CSC modulator/demodulator (modem) section 51a at the time of demanding a line, and for controlling the communication line modulator/demodulator (modem) section 44b so as to establish a frequency on an idle line in accordance with the instruction from the hub station 31 at the time of communication after the demand. Numeral 52 is an ODU (Out Door Unit) for transmitting into space radio signals outputted from the IDU 51 after amplifying those signals at the time of transmission, and for converting radio signals received from the satellite 33 into signals of an intermediate frequency and outputting those signals to the IDU 51, at the time of reception. Numeral 53 is an antenna.
The mentioned conventional system operates as described hereunder. First, in case of conducting a communication between a remote station 32a and another remote station 32b, the CSC modulator/demodulator (modem) section 51a of the remote station 32a modulates the frequency by the control of the IDU control section 51c to that of the forward CSC line 34 in response to a call from the terminal unit 50, and selects a time slot and transmits radio signals to the ODU 52 demanding a line connection (line assignment) to the hub station 31. The ODU 52 amplifies the mentioned radio signals and transmits them into space through the antenna 53.
The hub station 31 receives through the antenna 46 the signals demanding the line connection relayed through the satellite 33, and outputs the mentioned signals to transmitter/receiver 45. The transmitter/receiver 45 converts the signals demanding line assignment into intermediate frequency signals and outputs those signals to the CSC modulator/demodulator (modem) section 43. The CSC modulator/demodulator (modem) section 43 demodulates the signals demanding line connection and outputs them to the line connection control device 41. The DAMA control section 41a detects an idle communication line to be used for actual communication, and outputs a line assignment signal to the CSC modulator section 42. The mentioned signal is modulated at the CSC modulator 42, amplified at the transmitter/receiver 45, transmitted into space through the antenna 46, thus an available communication line (frequency and bandwidth to be used) is instructed to the remote stations 32a and 32b through the satellite 33.
Then, the remote stations 32a and 32b adjust the frequency of their respective modulator/demodulator (modems) sections 44b in tune with the instructed frequency band of the satellite communication line, and perform the communication. In case of conducting a communication between a remote station 32a and the hub station 31, the same steps as the foregoing are taken. That is, frequency of the communication line modulator/demodulator (modem) section 44a of the hub station 31 and that of the communication line modulator/demodulator (modem) section 44b of the remote station 32 are adjusted in tune with each other.
Supervision and control of each of the remote stations 32 by the hub station 31 is performed as described hereunder. The hub station 31 transmits a supervisory signal tuned to the frequency band of the backward CSC line 35 from the supervision and control section 41b into space through the CSC modulator 42, transmitter/receiver 45 and the antenna 46.
The supervisory signal received by a remote station 32 through the satellite 33 is modulated into an intermediate frequency at the ODU 52, demodulated at the CSC modulator/demodulator (modem) section 51a of the IDU 51, and then recognized as a supervisory signal destined to the remote station itself. At this time, if the remote station is normally operating, this remote station returns a reply signal to the hub station 31 through the CSC modulator/demodulator (modem) section 51a, the ODU 52 and the antenna 53, using the forward CSC line 34. The returned reply signal is recognized at the supervision and control section 41b in the line connection control device 41, whereby it is acknowledged that the remote station 32 is normally operating. The hub station 31 repeating the mentioned steps for each of the remote stations one after another conducts the supervision of all the remote stations 32.
In the mentioned conventional system, frequency bandwidth and information rate of the forward CSC line 34 were fixed at the time of demanding line connection and performing the supervision and control.
That is, in the foregoing prior art, the line assignment is performed regardless of data quantity to be transmitted. Since detection and assignment of a communication line is performed in response to each individual demand for line assignment, a problem exists in that in case of transmitting a small data quantity, the mentioned conventional communication system is time-consuming and inefficient. A further problem exists in that it is impossible to transmit data if there is no idle communication line.